The present invention relates to a device for detecting the state of rotation of a shaft, in particular for cassette tape recorders.
In tape recorders, be they of the recording or the playback type, it is very necessary to detect the state of rotation of a shaft which is part of the entrainment (drive) mechanism of the tape, so as to signal the end of the tape due to its complete unwindment from the take-off reel, or the stopping of the tape due to blockage of the drive mechanism or due to jamming. The need for the detection is even greater in the field of the cassette-type tape recorders, in which the tape unwinds from a take-off reel and winds on a take-up reel, while being completely enclosed in a cassette and therefore almost hidden from the view of the operator.
Additionally, a similar need is also felt in other related fields, for example, in the field of motion-picture cameras and projectors, in which once again there are used films enclosed in cassettes or, in any case, there are utilized instruments in which the movement of the film is hidden from the direct view of the operator.
There are generally known devices for detecting the state of rotation of a shaft to signal the end or the stoppage of a tape. Said devices are based on the transformation of the rotational motion of the shaft into a series of electrical pulses, obtained by means of a rotating switch, which are then used for the activation of an automatic means for blocking the tape drive motor or for ejecting the cassette tape carrier. Said electrical pulses, in the form of pulse trains, are generally obtained from a collector or commutator connected to the shaft to be monitored and being in the form of a segment-type collector or commutator on which there are located one or more brushes. An example of a device of this type and its use is described in Italian Pat. No. 849,991, which corresponds to U.S. Pat. No. 3,732,477 by R. Cicatelli. The pulse trains may be produced by means of magnetic and photoelectric devices.
All these known devices, however, have certain disadvantages: the segment-type collectors are subject to a notable wear, while the magnetic and photoelectric devices are very bulky.